1. Field of the Invention
The invention relations to the field of paper manufacturing and, more particularly, to a process and composition for delignifying a lignocellulosic material, such as chemical wood pulp, using a mixture of ozone and a peracid, particularly peroxymonosulfate.
2. Information Disclosure
Pulp is the raw material for the production of paper, paperboard, fiberboard and the like. In purified form, it is a source of cellulose for rayon, cellulose esters and other cellulosic products. Pulp is obtained from plant fiber such as wood, straw, bamboo and sugarcane residues. Wood is the source of 95% of the pulp fiber produced in the United States.
Dry wood consists of 40 to 50 percent cellulose, 15 to 25 percent other polysaccharides known as hemicelluloses, 20-30 percent lignin, a biopolymer which acts as a matrix for the cellulose fibers, and 5 percent of other substances such as mineral salts, sugars, fat, resin and protein. Lignin is composed primarily of methoxylated phenyl propane monomeric units interconnected by a variety of stable carbon-carbon and carbon-oxygen (ether) linkages. The lignin of conifers is apparently an oxidative polymerization product of coniferyl alcohol [3-(3'-methoxy-4'-hydroxyphenyl)allyl alcohol], while the lignin of deciduous trees appears to be derived from coniferyl alcohol and sinapyl alcohol [3(3',5'dimethoxy-4'-hydroxphenyl)allyl alcohol].
The strength of paper ultimately produced from pulp is dependent upon the chemical integrity of the cellulose, while the color arises from the lignin. The desired selectivity of a delignification process will be reflected in a low kappa number (little residual lignin) and a high viscosity of the residual pulp (little cleavage of long-chain celluloses).
Chemical pulp is manufactured by dissolving the lignin with hot solutions of (1) sodium hydroxide, (2) calcium, magnesium, or ammonium bisulfite, or (3) a mixture of sodium hydroxide and sodium sulfide (made from lime and reduced sodium sulfate). The products, known as soda pulp, sulfite pulp or sulfate (kraft) pulp, respectively consist of impure cellulose. In the chemical process, most of the hemicelluloses are also dissolved. Thus, the yield for chemical pulping is typically 40-60% based on wood weight. Mechanical pulps are characterized by their high yield and high lignin content. These pulps are called "mechanical" because a significant amount of mechanical energy (grinding and refining) is required to breakdown the wood chips. Chemical pulps contain about 5% lignin (weight basis) while mechanical pulps typically contain greater than 15% lignin. In order to make a white sheet from a chemical pulp almost all of the residual lignin must be removed. This is normally achieved by multistage bleaching using oxidants, some of which [chlorine (Cl.sub.2), chlorine dioxide (ClO.sub.2), and sodium hypochlorite (NaOCl)] contain chlorine. Presently, producers of bleached chemical pulp are seeking ways of decreasing or eliminating the use of chlorine-containing chemicals, the use of which leads to the formation and subsequent discharge of organochlorine compounds. Regulations to limit the discharge of adsorbable organic halogens (AOX) have already been established in several countries.
Replacement chemicals presently being used commercially or in research include oxygen, ozone, hydrogen peroxide and other peroxides. Oxygen is less selective than chlorine and chlorine dioxide and can therefore only be used for partial lignin removal. The cellulose is strongly affected, especially when the lignin content is low; therefore, the oxygen treatment must be of short duration.
Peroxymonosulfuric acid or Caro's acid and its caroate anions have features that are attractive for kraft pulp bleaching: 1) Caro's acid is a more efficient solubilizer of lignin than is H.sub.2 O.sub.2, 2) it is only marginally more expensive than H.sub.2 O.sub.2 because H.sub.2 SO.sub.4 is the only reactant needed to generate it from H.sub.2 O.sub.2, and 3) the sulfate anions in the resulting bleaching effluent can be recycled to the kraft recovery system. The use of Caro's acid for bleaching pulp is disclosed in U.S. Pat. Nos. 4,404,061; 4,475,984; 4,756,800; 4,773,966; 5,004,523 and European Patent 415 149. The formation reactions for Caro's acid and caroate anions are summarized in the following equations: EQU X H.sub.2 SO.sub.4 +H.sub.2 O.sub.2 .revreaction.H.sub.2 SO.sub.5 +(X-1) H.sub.2 SO.sub.4 +H.sub.2 O [1.] EQU H.sub.2 SO.sub.5 +H.sub.2 O+.revreaction.HSO.sub.5.sup.- +H.sub.3 O.sup.+ pKa&lt;O [2.] EQU HSO5.sup.- +H.sub.2 O.revreaction.SO.sub.5.sup.= +H.sub.3 O.sup.+ pKa=9.4 [3.]
The drawback of caroate delignification is the requirement of approximately 2.0 moles H.sub.2 SO.sub.4 per mole of H.sub.2 O.sub.2 to obtain a high yield of H.sub.2 SO.sub.5 in accordance with equation [1]. Using such a mole ratio, 17 kg of Na.sub.2 SO.sub.4 would be produced in a non-acidic effluent by the application of 2 kg of H.sub.2 O.sub.2 converted to Caro's acid or caroate. The kraft recovery system operates under alkaline conditions and requires approximately 17 kg Na.sub.2 SO.sub.4 /ton of pulp as make-up chemical. If a chemical balance is to be maintained without process modification, then the caroate charge would be limited to 2 kg equiv. H.sub.2 O.sub.2 /ton or 0.94 kg active oxygen (one active oxygen atom per hydrogen peroxide or caroate molecule). Fortunately, Na.sub.2 SO.sub.4 can be precipitated from the kraft recovery system. Still, even if process changes are made, the caroate charge is likely to be limited to approximately 0.25% active oxygen (A.O.) on pulp (1.77% HSO.sub.5.sup.-). To design a reaction tower and washing system for such a low oxidant charge is not economically attractive. Moreover the delignification that is possible with an upper limit of 0.25% active oxygen from caroate is not adequate for many purposes.
Ozone delignification is well known in the art (see U.S. Pat. No. 5,164,044 and references therein). Its major drawback is that both ozone itself and hydroxyl radicals generated from the ozone attack cellulose along with lignin. Hydroxyl radicals are relatively unselective toward lignin in the presence of cellulose. The result is that it is recommended that the ozone charge be limited to 1% of the pulp weight to avoid significant loss in strength of paper made from ozone-delignified pulp. [see Patt et al. Das Papier 42, V14-V23 (1988)].
There is thus a need for a delignification process that is highly selective for lignin with minimal concomitant degradation of cellulose polymers.
There is a further need for a process for bleaching chemical wood pulp that minimizes or avoids the use of chlorine-based bleaching agents while at the same time not creating waste disposal problems of its own.